The present invention relates to a semiconductor laser device of a group III-V nitride semiconductor represented by a general formula, AlxGayIn1xe2x88x92xxe2x88x92yPvAswN1xe2x88x92vxe2x88x92w (wherein 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, x+yxe2x89xa61, 0xe2x89xa6vxe2x89xa61, 0xe2x89xa6wxe2x89xa61 and v+wxe2x89xa61), which shows laser action with a wavelength ranging from the blue region to the ultraviolet region and is expected to be applied to fields of optical data processing and the like.
Recently, large capacity optical disk systems such as a digital video disk system have been put to practical use, and the recording capacity of an optical disk is now being further increased. As is well known, for the purpose of increasing the recording capacity, it is one of the most effective means to shorten a wavelength of a laser beam used as a light source for recording or reproducing information. A semiconductor laser chip included in an existing digital video disk system is made from a semiconductor material mainly including AlGaInP among group III-V semiconductor materials, and has a wavelength for laser action of 650 nm. Accordingly, a laser device with a shorter wavelength using a group III-V nitride semiconductor material is indispensable for a high density digital video disk system now under development.
Now, a conventional group III-V nitride semiconductor laser device will be described with reference to a drawing.
FIG. 8 shows a sectional structure of the conventional group III-V nitride semiconductor laser device.
As is shown in FIG. 8, a buffer layer 102 of GaN and an n-type contact layer 103 of n-type GaN with low resistance are successively formed on a substrate 101 of sapphire. In an element region on the n-type contact layer 103, an n-type cladding layer 104 of AlGaN, an n-type light guiding layer 105 of n-type GaN, a multiple quantum well active layer 106 including alternately stacked well layer of Ga1xe2x88x92xInxN and barrier layer of Ga1xe2x88x92yInyN (wherein 0 less than yxe2x89xa6x less than 1), a p-type light guiding layer 107 of p-type GaN and a p-type cladding layer 108 of p-type AlGaN having a ridge stripe portion 108a in the shape of a ridge with a width of 3 through 10 xcexcm on the top surface thereof are successively formed.
On the p-type cladding layer 108, a p-type contact layer 109 of p-type GaN with low resistance is formed, and on the p-type contact layer 109, a p-side electrode 110 is selectively formed. The top surface of the p-type cladding layer 108 on both sides of the ridge stripe portion 108a excluding the p-side electrode 110 and the side surfaces of the element region are covered with an insulating film 111. On the insulating film 111, a wire electrode 112 is formed so as to be in contact and cover the p-side electrode 110, and an n-side electrode 113 is formed on the n-type contact layer 103 on a side of the element region.
When the semiconductor laser device having the aforementioned structure is grounded at the n-side electrode 113 and supplied with a given voltage at the wire electrode 112, the semiconductor laser device shows laser action with a wavelength of 370 nm through 430 nm. This wavelength for laser action is varied depending upon the compositions and the thicknesses of the layers of Ga1xe2x88x92xInxN and Ga1xe2x88x92yInyN included in the multiple quantum well active layer 106. At present, continuous laser action has been achieved at a temperature exceeding room temperature and will soon be put to practical use. However, in order to increase the recording capacity of an optical disk system, a semiconductor laser device capable of showing laser action with a shorter wavelength is desired to realize.
However, in the conventional group III-V nitride semiconductor laser device, the wavelength for laser action cannot be made shorter than approximately 370 nm and is difficult to further shorten in view of the operating principle.
In order to realize a semiconductor laser with a shorter wavelength, a so-called wide gap semiconductor having a wide band gap (energy gap) is used as an active layer. For example in the aforementioned multiple quantum well active layer 106, a shorter wavelength can be attained by using, as the well layer, Ga1xe2x88x92xInxN with the composition ratio of In of 0, namely, GaN, or AlGaN including Al for further widening the energy gap.
In a double heterostructure laser device in which carriers and produced light are confined in an active layer, a semiconductor material having a wider energy gap than the active layer is required to be used as a cladding layer.
In general, in order to obtain a semiconductor laser device having practical operation characteristics operable at room temperature or more, it is necessary to use a cladding layer having an energy gap wider than that of an active layer by at least approximately 0.4 eV. Since the energy gap of a semiconductor of AlGaN can be widely changed in a range between 3.4 eV and 6.2 eV, it is possible to form a cladding layer with a wide energy gap. However, when the semiconductor of AlGaN has a composition with a wide energy gap, p-type impurity doping for obtaining a p-type semiconductor becomes particularly difficult because the thermal activation efficiency of holes is lowered. Therefore, at present, merely a p-type semiconductor with the composition ratio of Al of approximately 0.2 at most (namely, Al0.2Ga0.8N as a mixed crystal) and with an energy gap of approximately 4.0 eV at most can be obtained.
The present inventors have extensively examined the reason for which a p-type group III-V nitride semiconductor, particularly a semiconductor of p-type AlGaN, can merely attain an energy gap up to approximately 4.0 eV at most, resulting in reaching the following conclusion:
FIG. 9 shows energy levels of p-type gallium nitride (GaN) and p-type aluminum nitride (AlN), wherein the ordinate indicates the energy of electrons. As is shown in FIG. 9, above valence bands Ev of GaN and AlN, an acceptor level Ea derived from magnesium (Mg) working as a p-type dopant is formed. Mg is generally regarded as an acceptor that is the shallowest in a nitride semiconductor, namely, that has the lowest binding energy and can be easily activated, and hence is widely used as a p-type dopant.
However, even Mg has a comparatively high acceptor level of 0.15 eV from the energy Ev at the upper end of the valance band of GaN. As is well known, a thermal energy corresponding to room temperature is approximately 0.025 eV, and the thermal activation efficiency of Mg at room temperature is merely approximately 1%. Accordingly, in order to obtain a carrier concentration of 1xc3x971017 cmxe2x88x923 through 1xc3x971018 cmxe2x88x923 required in a p-type cladding layer, the dope concentration of Mg should be 1xc3x971019 cmxe2x88x923 through 1xc3x971020 cmxe2x88x923. The dope concentration of Mg of 1xc3x971020 cmxe2x88x923 approximates to a limit for obtaining a good semiconductor crystal, and when Mg is further doped, the crystallinity becomes very poor. Accordingly, with a carrier concentration attained by the impurity concentration of 1xc3x971020 cmxe2x88x923 regarded as a limit of the dope concentration, it is necessary to attain a thermal activation efficiency of the acceptor of 0.1% or more in order to obtain a carrier concentration exceeding 1xc3x971017 cmxe2x88x923.
On the other hand, as is shown in FIG. 9, the acceptor level Ea of Mg is deeper in AlN, and reaches approximately 0.6 eV. For example, in AlyGa1xe2x88x92yN, the acceptor level is substantially linearly changed from 0.15 eV to 0.6 eV by changing the composition ratio y of Al. In order to attain the thermal activation efficiency of the acceptor of 0.1% or more, it is necessary to make comparatively small a difference between the acceptor level Ea and the energy Ev at the upper end of the valence band, and hence, the composition ratio y of Al cannot be increased.
When the composition ratio of Al cannot be thus increased, the proportion of electrons that are not recombined with holes but leak to the p-type cladding layer becomes large among the electrons injected from the n-type cladding layer into the active layer. As a result, the proportion of holes that leak to the n-type cladding layer becomes large among the holes injected from the p-type cladding layer into the active layer. Such a leakage current does not contribute to laser action, and hence increases a threshold current for laser action. Furthermore, when a semiconductor laser device is operated at a high temperature, the proportions of the electrons and holes leaking from the active layer are further increased. Therefore, the threshold current is largely increased in accordance with increase of the temperature, resulting in degrading the temperature characteristic of the semiconductor laser device.
Moreover, when a crystal having a large composition ratio of Al is stacked on a crystal having a small composition ratio of Al, a stress is caused due to a difference in the lattice constant therebetween. In particular, such a stress can cause a crack in a cladding layer required to have a thickness of 1 xcexcm or more, resulting in degrading the laser characteristic and the reliability.
In consideration of the aforementioned conventional problems, an object of the invention is realizing a semiconductor laser device that has a low threshold current even in the ultraviolet region and has an excellent temperature characteristic.
In order to achieve the object, the nitride semiconductor laser device of this invention includes a p-type barrier layer formed between an active layer and a p-type cladding layer for preventing electrons from leaking from an n-type cladding layer to the p-type cladding layer, and an n-type barrier layer formed between the active layer and the n-type cladding layer for preventing holes from leaking from the p-type cladding layer to the n-type cladding layer.
Specifically, the first semiconductor laser device of this invention comprises an n-type cladding layer of an n-type first nitride semiconductor formed on a substrate; an active layer, formed on the n-type cladding layer, of a second nitride semiconductor having a narrower band gap than the first nitride semiconductor; a p-type cladding layer, formed on the active layer, of a p-type third nitride semiconductor having a wider band gap than the second nitride semiconductor; and a p-type barrier layer, formed between the active layer and the p-type cladding layer, of a p-type fourth nitride semiconductor having a wider band gap than the first nitride semiconductor.
In general, when the wavelength for laser action ranges over the ultraviolet region, the n-type cladding layer is required to have a band gap (energy gap) of at least approximately 4.4 eV. Therefore, in electrons injected from the n-type cladding layer into the active layer, the amount of electrons not injected into the active layer having a smaller thickness than the cladding layer but leaking to the p-type cladding layer is increased. However, since the first semiconductor laser device comprises the p-type barrier layer having a wider band gap than the n-type cladding layer and formed between the active layer and the p-type cladding layer, the electrons that are otherwise not injected into the active layer but leak to the p-type cladding layer can be effectively injected into the active layer. Accordingly, the threshold current does not increase, resulting in attaining an excellent operation characteristic.
The second semiconductor laser device of this invention comprises a p-type cladding layer of a p-type first nitride semiconductor formed on a substrate; an active layer, formed on the p-type cladding layer, of a second nitride semiconductor having a narrower band gap than the first nitride semiconductor; an n-type cladding layer, formed on the active layer, of an n-type third nitride semiconductor having a wider band gap than the second nitride semiconductor; and a p-type barrier layer, formed between the p-type cladding layer and the active layer, of a p-type fourth nitride semiconductor having a wider band gap than the third nitride semiconductor.
Although the second semiconductor laser device includes the p-type cladding layer formed on a surface of the active layer closer to the substrate, the p-type barrier layer having a wider band gap than the n-type cladding layer is formed between the p-type cladding layer and the active layer. Therefore, the electrons otherwise leaking to the p-type cladding layer can be effectively injected into the active layer as in the first semiconductor laser device.
In the first or second semiconductor laser device, the p-type cladding layer preferably includes phosphorus or arsenic. In this manner, even when the p-type cladding layer includes aluminum (Al) in a comparatively large amount for widening the band gap, a difference between an energy at the upper end of the valence band and an energy of the acceptor level can be prevented from increasing in the p-type cladding layer with keeping a wide energy gap of the p-type cladding layer. In other words, the acceptor level of the p-type cladding layer can thus be lowered, and hence, the p-type cladding layer can be doped with a p-type dopant in a desired manner. As a result, the p-type cladding layer can attain a band gap sufficiently wide to be applied to the active layer having a band gap capable of emitting violet light.
In this case, the p-type cladding layer preferably has a composition for attaining a lattice constant substantially according with a lattice constant of gallium nitride, a lattice constant of a nitride semiconductor layer formed on a surface of the active layer closer to the substrate or a lattice constant of the substrate. In this manner, even when the p-type cladding layer includes Al in a comparatively large amount for widening the band gap, the lattice constant of the p-type cladding layer can be prevented from reducing. As a result, the crystallinity of the p-type cladding layer required to have a comparatively large thickness can be improved.
In the first or second semiconductor laser device, the p-type barrier layer preferably includes phosphorus or arsenic. In so doing, the energy at the upper end of the valence band and the energy at the lower end of the conduction band in the p-type barrier layer shift upward, so that a potential barrier to the hole is further reduced, with a result of efficient injection of the holes to the active layer.
In this case, the p-type barrier layer preferably has a composition for attaining a lattice constant substantially according with a lattice constant of gallium nitride, a lattice constant of a nitride semiconductor layer formed on a surface of the active layer closer to the substrate or a lattice constant of the substrate.
In the first or second semiconductor laser device, the p-type barrier layer preferably has a thickness of 1 nm or more and 100 nm or less. Thus, the p-type layer can be in a thickness for decreasing a tunnel probability of electrons without decreasing a tunnel probability of holes.
In the first or second semiconductor laser device, the active layer and the p-type barrier layer are preferably adjacent to each other. When the p-type barrier layer and the active layer are adjacent to each other, the p-type barrier layer can definitely work as an energy barrier against the electrons injected from the n-type cladding layer but not injected into the active layer.
The first or second semiconductor laser device preferably further comprises, between the p-type barrier layer and the p-type cladding layer, a p-type carrier injection layer of a p-type fifth nitride semiconductor having a band gap wider than the band gap of the active layer and narrower than the band gap of the p-type cladding layer. In this manner, holes having comparatively high energy injected from the p-type active layer once drop into the p-type carrier injection layer having a narrower band gap than the p-type cladding layer, so as to slightly reduce their energy, and the holes are then injected into the active layer. Accordingly, the efficiency of injecting holes into the active layer can be improved, resulting in further improving the operation characteristic of the laser device.
In this case, the p-type carrier injection layer preferably includes phosphorus or arsenic.
Furthermore, in this case, the p-type carrier injection layer preferably has a composition for attaining a lattice constant substantially according with a lattice constant of gallium nitride, a lattice constant of a nitride semiconductor layer formed on a surface of the active layer closer to the substrate or a lattice constant of the substrate.
Moreover, the active layer and the p-type barrier layer are preferably adjacent to each other, and the p-type barrier layer and the p-type carrier injection layer are preferably adjacent to each other.
The third semiconductor laser device of this invention comprises an n-type cladding layer of an n-type first nitride semiconductor formed on a substrate; an active layer, formed on the n-type cladding layer, of a second nitride semiconductor having a narrower band gap than the first nitride semiconductor; a p-type cladding layer, formed on the active layer, of a p-type third nitride semiconductor having a wider band gap than the second nitride semiconductor; an n-type barrier layer having a wider band gap than the third nitride semiconductor and formed between the n-type cladding layer and the active layer; an n-type carrier injection layer having a band gap narrower than the band gap of the first nitride semiconductor and wider than the band gap of the second nitride semiconductor and formed between the n-type cladding layer and the n-type barrier layer; a p-type barrier layer having a wider band gap than the first nitride semiconductor and formed between the active layer and the p-type cladding layer; and a p-type carrier injection layer having a band gap narrower than the band gap of the third nitride semiconductor and wider than the band gap of the second nitride semiconductor and formed between the p-type barrier layer and the p-type cladding layer.
The third semiconductor laser device includes, in addition to the p-type barrier layer for reflecting electrons injected from the n-type cladding layer on a side of the p-type cladding layer, the n-type barrier layer for reflecting holes injected from the p-type cladding layer on a side of the n-type cladding layer. The third semiconductor laser device further includes the p-type carrier injection layer for improving the efficiency of injecting holes into the active layer formed between the p-type barrier layer and the p-type cladding layer, and the n-type carrier injection layer for improving the efficiency of injecting electrons into the active layer formed between the n-type barrier layer and the n-type cladding layer. Accordingly, the threshold current can be prevented from increasing, resulting in stably emitting a laser beam with a short wavelength ranging over the ultraviolet region.
In the third semiconductor laser device, at least one of the n-type cladding layer and the p-type cladding layer preferably includes phosphorus or arsenic.
In this case, at least one of the n-type cladding layer and the p-type cladding layer preferably has a composition for attaining a lattice constant substantially according with a lattice constant of gallium nitride, a lattice constant of a nitride semiconductor layer formed on a surface of the active layer closer to the substrate or a lattice constant of the substrate.
In the third semiconductor laser device, at least one of the n-type barrier layer and the p-type barrier layer preferably includes phosphorus or arsenic.
In this case, at least one of the n-type barrier layer and the p-type barrier layer preferably has a composition for attaining a lattice constant substantially according with a lattice constant of gallium nitride, a lattice constant of a nitride semiconductor layer formed on a surface of the active layer closer to the substrate or a lattice constant of the substrate.
In the third semiconductor laser device, each of the n-type barrier layer and the p-type barrier layer preferably has a thickness of 1 nm or more and 100 nm or less.
In this case, at least one of the n-type carrier injection layer and the p-type carrier injection layer preferably includes phosphorus or arsenic.
In this case, at least one of the n-type carrier injection layer and the p-type carrier injection layer preferably has a composition for attaining a lattice constant substantially according with a lattice constant of gallium nitride, a lattice constant of a nitride semiconductor layer formed on a surface of the active layer closer to the substrate or a lattice constant of the substrate.
In the third semiconductor laser device, the active layer and the n-type barrier layer are preferably adjacent to each other, and the n-type barrier layer and the n-type carrier injection layer are preferably adjacent to each other.
In the third semiconductor laser device, the active layer and the p-type barrier layer are preferably adjacent to each other, and the p-type barrier layer and the p-type carrier injection layer are preferably adjacent to each other.